Nuclear-Radiological (hereunder occasionally referred to as “nuc/rad” or SNM/RDD) Threat Screening Portals have been developed starting with the Manhattan project (1942-1946). These systems were developed mainly to ensure workplace and product safety in the nuclear industry with its estimated 1,000 nuclear facilities worldwide and in the scrap metal industry.
Since the early 1990s, industry and national labs worldwide expanded research efforts to modify existing screening systems so they can detect illicit trafficking of nuclear-radiological materials, nuclear devices and RDD (Radiological Dispersal Device) as well.
The present invention relates generally to Special Nuclear Materials (SNM) and Radiological Dispersion Devices (RDD) Screening Portals (NRSP).
An extensive review of the prior art is given in U.S. provisional application 60/654,964 referred to in the related applications section and incorporated herein by reference. Nearly all the systems described in the Prior Art chapter do not meet, to a great extent, the challenge of detecting many nuclear/radiological threats. Furthermore, existing Nuclear-Radiological Threat Detection Portals have a more limited detection performance in terms of overlooked nuc/rad threat detection and false alarm rate against sophisticated terror-related attacks and trafficking that homeland security authorities would like to intercept in the post 9/11 period.
To better understand Nuclear/Radiological Portals technologies, two prior art examples of checkpoint operational flow charts (see FIGS. 1 and 2) are presented. These demonstrate a typical process implemented when a person or a parcel or a shipment, or a vehicle or other conveyance (such as a boat or train) with a nuclear-radiological threat passes through a nuclear-radiological detection portal.
As shown in FIG. 1, the process entails a rather complex and expensive multi-stage screening procedure that results either in an “all clear” decision by the portal supervisors or in seizure of the cargo (or person) by the relevant authorities.
The security forces (or Hazmat, NEST teams, first-responders, or nuclear regulatory, law enforcement bodies) operational flow chart is shown in FIG. 1. This is a generalized flow chart that delineates the complex inter-relation between the technical aspects of detection at a checkpoint (e.g., NEST, border or roadside checkpoint) and the various organizations involved (e.g., nuclear experts, police) once a suspected item turns the alarm on.
The local supervisor of a nuclear/radiological portal has a different procedure to follow (see FIG. 2). The main mission is to reconcile three conflicting demands:                Detect “all” nuclear/radiological threats        Reduce false alarms to a minimum        Enable uninterrupted traffic flow        
Prior Art nuclear/radiological portals encounter several difficulties that limit their detection performance:                Natural background radiation (NORM). Prior art detectors are generally not collimated, in order to have a large field of view encompassing more possible threats. Since their field of view often approaches 180°, they detect a large amount of radiation (for example, environmental radiation, cargo scatter and other sources) from sources spatially remote from the loci of threats.        Radiation emanating from benign sources, for example goods which have NORM, medical isotopes and industrial isotopes.        SNM (Special Nuclear Materials)—Weapon grade uranium (WgU) and plutonium WgP) emit a low rate of gamma rays and neutrons. This makes it difficult to detect them (especially when shielded).        Limited sensitivity of detection.        Radiation sources concealed in “radiation-shielded” means (e.g., lead and/or cargo and or neutron moderators) which attenuate the detected activity of gamma and neutron particles.        
Natural environmental background radiation can impede the detection of low activity and/or shielded SNM/RDD threats, and it is also a potential source of false alarms. Natural background radiation emanates from both cosmic and terrestrial radiation. Natural background radiation level (FIGS. 3 and 4) depends on a variety of uncontrolled factors, such as geographic location, ground level, masking by passing objects like vehicles, rain and the random nature of such radiation.
To reduce the effects of environmental background radiation, various algorithms and nuclear electronic means are currently used, designed to detect low intensity radiation threats while keeping random false alarms at an acceptable level. To further reduce background radiation, the system's detectors are lead-shielded on the sides that do not face the object under screening.
As indicated above, radiation emanating from benign sources might cause false alarms. There are two distinct groups of benign radiation sources that may cause false alarms:                Nuclear medicine isotopes, and patients who were the recipients of such isotopes.        Benign commercial goods and materials.        Normally occurring radioactive materials and other background radiation (NORM). Some of these are further discussed in the following sections.Nuclear Medicine Radiopharmaceuticals        
Every year, about 40 million patients worldwide (including about 17 million in the U.S.) undergo some type of a nuclear medicine procedure. Most radio pharmaceutics in use decay within 5-10 days. During this period, such patients and/or the benign delivery of radio pharmaceutics may trigger a false alarm in nuclear/radiological portals.
The following isotopes comprises of >98% of all clinical procedures in nuclear medicine: Gallium-67; Technetium-99m; Thallium-201; Iodine-123; Iodine-125; Iodine-131; Xenon-133; and Indium-111
Note: Tc99 (140 Kev) constitutes more than 85% of all nuclear medicine applications. Tc99 has a half-life (rate of decay) of 6 hours. Note that there are more isotopes used in NM not listed in the table.
At any given time, in North America alone, it is estimated that approximately one in 2,500 people emit gamma radiation that may trigger an alarm at a nuclear/radiological portal, unless advanced identification means and methods are used to detect and screen out such cases.
Benign Materials and Goods Containing Natural Radioactivity (Norm)
Many benign goods emit radioactive radiation. The most frequent naturally occurring radio nuclides are K40, natural U226, Ra226 and natural Th232.
Table 1 below shows the typical natural activity emission in [Bq /Kg]
TABLE 1Benign Materials Containing Naturally Occurring RadioactivityTypical Activity Concentration in Bq/Kg−SubstanceK40Ra226Th232Adobe200-200010-10020-200Concrete100-500 30-60 30-50 Feldspar1500-5000 30-10050-200Fertilizers 30-10000 10-100010-40 Granite500-500025-50030-100Marble30-20010-40 15-30 Monazite Sand30-100 20-1000 40-4000Sandstone100010-10010-100Slate500-100020-80 30-80 
It is known that common goods, like: decorated glass, dental ceramics, marijuana, polishing powder, bananas, consumer goods lenses, and Thorium glass compounds, may trigger an alarm at a nuclear/radiological portal. To date, the preferred technology to reduce false alarms emanating from benign sources is to use Gamma spectroscopy that identifies some benign isotopes. Most presently deployed portals do not include such means, due to cost considerations. Those that do, use only such means for gamma identification.
In order to identify a threat, a sufficient number of neutron and/or Gamma particles must interact with a portal's detectors and be recorded by the system electronics. This requires a high sensitivity detection medium (e.g., G.M. counters have a much lower sensitivity than scintillation crystals), and a close proximity between the detectors and the item under surveillance (e.g., vehicle, enclosure).
Nuclear Gamma radiation is absorbed by high Z materials (e.g., lead, steel). Neutrons are attenuated by low Z elements (e.g. water). It must be assumed that terrorists may attempt to “shield” nuclear/radiological threats in a radiation-absorbing “enclosure”.
Highly Enriched Uranium (HEU) Special Nuclear Materials (SNM) detection poses a substantial challenge, since HEU emits a very low rate of spontaneous fission, neutrons, and most of the gamma emission is at low energy which is easily attenuated by cargo and shields.
In order to detect some types of shielding it is possible to develop an electro-magnetic metal detector that will have high detection efficiency for uranium, lead, and plutonium detection. There are currently at least two companies that offer a link to an optional metal detector.
Weapons-Grade Plutonium (WgP) Detection
The leading detection means for weapons-grade plutonium are neutron detectors (e.g., He3, glass fiber). This is due to three properties:                1. Low natural neutron background radiation        2. Relatively high flux of neutrons (950-200 neutrons per kg per sec)Weapons-Grade Uranium (WgU) Detection        
The leading prior art means of detecting WgU is by detecting the gamma emission of WgU. U235 emits some 1.01 MeV gamma rays and reactor recycled WgU includes traces of U232 which via its T1206 product emits 2.6 MeV gamma rays.
Due to its low rate of spontaneous fission neutrons, neutron detection is not used in the prior art to detect WgU.
Detection of “Radiation Dispersal Devices” (RDD)—Prior Art Technologies
For SNM-based RDD, the same technologies used for SNM detection are used.
For gamma-emitting radioisotopes RDDs (e.g., Co60, Cs137), the main obstacles to detection are:                1. Potential lead shielding (which is fairly easy to implement for isotope energies below 500 KeV).        2. Nuclear medicine (N.M.) and NORM false alarms (note that 99% of N.M. isotopes are in the 80-300 KeV range).Propr Art Radiation Detectopm Portal Description—By Technology and Application        
To assist in focusing on the relevant Prior Art, we will describe here Prior Art technologies from two different perspectives:    1) Current Core Detection Technologies segments:            Gamma Detectors        Neutron Detectors        Neutron Activation Detection            2) Detection Systems By Application:            People Screening Portals        Train Screening Portals        Vehicle Screening Portals        Enclosure Screening Cranes        Parcels & PackagesComparison of Prior Art Nuclear/Radiological Portals Core (Detection) Technologies        
FIG. 5 provides a roadmap of the various core passive detection technologies currently used to detect nuclear radiological materials and devices trafficking.
There are two classes of detection technologies:                Direct radiation detectors—These detectors convert radiation particles energy directly into electrically-charged impulses that are processed by the system's electronics (e.g., HPG, Geiger counters and He3 proportional counters, CZT).        Scintillators—This subgroup of detectors (e.g., organic and inorganic scintillators) functions in two steps: the energy of the particles is converted into light (photon). The flash of light scintillation is picked-up and converted into an electrical impulse by a photo detector (e.g., photomultiplier).        
The following Table 2 describes some of the more important aspects distinguishing each of the detectors used in nuclear/radiological portals.
The two tables below (Tables 2 and 3) illustrate the reason most portal designers elected in the past to use plastic scintillators for Gamma-only portals, and He3 filled detectors for Neutron detection, since these two detection means provide optimal cost-performance for systems developed. At present the leading spectroscopy based portals detectors are NaI(T1) and HPG. In the following tables, 1 is poor, 5 is excellent.
TABLE 2Prior Art Radiation Detection Technologies- Major Properties ComparisonEnergyStoppingIsotopeTechnologyCostResolutionPowerI.D.Geiger Counters5No1NoPlastic Scintillators50 to 12NoNal (Ti), Csl(TI),1443BGO Scintillators:Isotope I.D. ModeHe3 Detector2233Proportional He33243Detector ArrayNeutron3233“Glass Detectors”Neutron ActivationVeryN/A55Plutonium & HEUHighDetection & ImagingCost
Two typical configurations of Geiger tube portals are shown in FIG. 6. Such portals were developed in the early 1950's for nuclear safety applications. Since in safety applications any detection of radiation (above a background level) should trigger an alarm, it exhibited satisfactory cost performance.
At present, only a handful of security portals use Geiger detectors.
TABLE 3Geiger Counters - Advantages and DisadvantagesAdvantagesDisadvantagesExtremely low costPoor sensitivity (low Z results inSimple and low cost electronicslimited stopping power)Proven technologyHigh rate of false alarmsSimple maintenancePoor detection of high energy (e.g.,Co60) radiation sourcesLimited Count Rate Performance
Large (2-5 cm thick) slabs of plastic scintillators blocks are the primary detector used extensively in nuclear/radiological portals.
Various plastic materials such as Anthracene emit extremely short (2-5 nanoseconds) scintillations of light when Alpha, Beta, Neutron or Gamma particles interact with the plastic molecule (via a photoelectric and multiple Compton effects). These short light scintillation are picked up and amplified by a photomultiplier tube (FIG. 8).
The “train” or string of impulses is further amplified by a low-noise amplifier and the pulse is registered by digital electronics (not shown). When the rate surpasses the rate of Normal Background Radiation (NBR) at a statistically meaningful level, an alarm is triggered.
The prior art thinking is that since plastic scintillators do not provide energy resolution and thus cannot provide spectroscopy isotope ID, such detectors result in a high rate of false and nuisance alarms.
In spite of their inherent limitations (see Table 4), plastic scintillators are used extensively in present-day nuclear/radiological portals, mainly due to the low cost of large volume (5-50 liter) plastic scintillators as detectors without effective energy resolution.
TABLE 4Plastic Scintillators - Advantages and DisadvantagesAdvantagesDisadvantagesProven technologyHigh rate of false alarmsEase of fabrication in various shapesRequires periodic calibrationand volumesHigh rate of overlooked threatsRobust MaterialNo energy spectroscopyLow costSimple to maintainScintillation (Inorganic) Crystals (e.g., Nal (Ti), Csl (TI), BGO)
When Gamma particles interact with a mono-crystalline scintillator material, ionized (excited) atoms in the scintillator material “relax” to a lower-energy state and in the process emit a scintillation of photons. In a scintillator crystal, the return of the atom to lower-energy states with the emission of photons is an inefficient process. Furthermore, the emitted photons usually have a high energy which generates photons that do not lie in the range of wavelengths to which the PMT is sensitive. To enhance the emission of visible photons, small amounts of impurities (called activators) are added to most scintillators. The crystal's de-excitations, channeled through these impurities, give rise to photons that activate the PMT (see FIG. 8).
The light pulses are converted to an electrical impulse and amplified by the photomultiplier and a low noise amplifier. As the intensity of each pulse is proportional to the Gamma energy of the primary particles, a threshold device is inserted to reject low energy background radiation.
The most popular scintillators used in nuc/rad portals are:                NaI (T1)—Sodium Iodide Thallium activated crystals        BGO—Bismuth—Germanium—Oxygen Crystals        
TABLE 5Inorganic Scintillators - Advantages and DisadvantagesAdvantagesDisadvantagesProven technologyLimited sensitivity - due to use ofUpgradeable to providesmall detectorsIsotope I.D.Sensitive to Environmental ConditionsHigh CostNal (TI), Csl (Ti), BGO Scintillators & Isotope Spectroscopic I.D.
These scintillation crystals have a moderate energy resolution (5%-14% depending on scintillator type and energy), enabling the addition of isotope I.D. electronics, reducing the false alarm rate dramatically.
By digitizing each impulse (via an A/D converter) and sorting it in a spectrometer, for example a multi-channel analyzer, an I.D. of the radioisotope can be achieved (FIG. 9).
For example, if the spectrometer reading shows that the Gamma source energy is at 140 KeV±3%, then the portal threat identification can assume that this is a benign radiation of Tc99 used routinely in nuclear medicine.
TABLE 6Nal (Tl), Csl (Ti), BGO Scintillators & Isotope I.D. (Multi-ChannelAnalyzer) - Advantages and DisadvantagesAdvantagesDisadvantagesProven technologyLow sensitivitySpectroscopic IsotopeSensitive to ambient temperatureI.DHigh costHe3 Neutron Detectors
The main function of neutron detectors in nuclear/radiological portals is to detect WgP.
Neutrons have no electrical charge but have a considerable mass. As such, they cannot produce an electrical charge (ionization) directly. Neutron detection relies on the interaction of neutrons with matter. Such an interaction produces a secondary charged particle with a charge proportional to the original neutron energy (this is why such detectors are called “Proportional Detectors”).
There are many alternative designs of He 3neutron detectors, but all of them comprise a metal enclosure filled with He3 gas (FIG. 10), two electrodes (anode and cathode) and charge detection electronics (the reaction used is indicated as n+He3→P+He3+765 KeVs).
Neutron detectors are efficient for low-energy neutrons (Thermal Neutrons) and inefficient for the detection of high energy (“fast”) neutrons. Because of this phenomena “moderators” that “slow down” the fast neutrons are used. The electronics associated with the detector cannot determine the original neutron energy, which is “lost” due to the moderator.
A limiting factor in He3 neutron detectors, used in nuclear/radiological portals for the detection of fissile plutonium and more so for HEU detection, is the limited sensitivity of single He3 detectors.
Glass Fiber Neutron Detectors
Glass, or recently developed “glass fiber”, detectors are used in some advanced neutron-detection portals, due to their robustness and semi-imaging capabilities.
The beam of fast neutron (FIG. 11) is slowed down by a moderator and creates Thermal Neutrons (TN). Thermal neutron flux impinges on the core of a glass fiber optic. In one example, this core is composed of glass with embedded Li6, and Ce3.
The thermal neutron is captured by the Li6, through a nuclear process. Alpha and He3 particles are generated. The He3 particles excite the Ce3, which generates light photons. The photons travel along the fiber optic device and are detected by a photomultiplier (not shown) that amplifies the signal and creates a nanosecond electrical charge impulse that is fed to detection electronics.
TABLE 7Comparison Between Prior Art He3and Glass Fiber Neutron DetectorsTechnologyAdvantagesDisadvantagesHe3 DetectorsProven matureLimited detection of hightechnologyneutron fluxCan be produced in anyLow Sensitivityshape or sizeMechanical microphonicsTime-of-flight measurement(TOF) not enabledNeutron spectroscopy notenabledGlass FiberSolid state - robustTechnology is not matureDetectorsMaintenance freeShape & size limitationCan be used for highSensitivity limitationneutron fluxCan't measure neutronIts spatial resolutionenergyprovides an imagingoptionPrior Art Limitations
Whether it uses spectroscopic or non-spectroscopic detection and identification methods, the prior art has numerous limitations, some of which are:                1) Benign radiation is a major limiting factor of ASP (Advanced Spectroscopic Portal) threat detection performance. Background, defined as NORM medical and industrial sources, its direct radiation and scatter and x-radiation, is a major limiting factor to the achievement of premium SNM and RDD threat detection.        2) Conventional spectroscopy is detector sensitivity limited in real world cases, due to limited number of spectra superimposed on background radiation and other benign sources.        3) Limitation in energy resolution limits spectroscopic identification.        4) The prior art cannot deliver extremely low false alarm and low overlooked threat rates required for realistic development and operation of NRSP's. Achieving a true alarm rate of >99.6%, and a false alarm rate of <1:10000 vehicles is not possible using prior art techniques.        5) Manufacturing costs are a limiting factor for wide scale deployment of NSRP systems.        6) Multi-detector spectroscopy isotopes identification is complicated due to sensitivity to environmental conditions.        7) Prior art spectroscopic isotope identification algorithms may fail to identify isotopes in complex spectra.        8) Prior art technology is detection sensitivity limited.        9) The Prior Art does not detect other threats (e.g. explosives, bio-chemical agents).        10)Low throughput—The prior art is generally limited to slow moving objects, for example, vehicles moving at 5 MPH. It does not perform at vehicle cruising speeds of greater than 20 MPH.        11) The prior art requires two types of detectors one for gamma and one for Neutrons.        
While the present invention does not ameliorate all of these limitations, some embodiments of the invention deal with one or more of them.